William K. W. Li

Autotrophic Picoplankton in the Tropical Ocean

In phytoplankton of the eastern tropical Pacific Ocean from 25 to 90 percent of the biomass (measured as chlorophyll a) and 20 to 80 percent of the inorganic carbon fixation were attributable to particles that could pass a screen with a 1-micrometer pore diameter. Evidence is presented that these are indeed autotrophic cells and not cell fragments.

Smallest Algae Thrive As the Arctic Ocean Freshens

In the Arctic Ocean, phytoplankton cell sizes have decreased with warming temperatures and fresher surface waters. As climate changes and the upper Arctic Ocean receives more heat and fresh water, it becomes more difficult for mixing processes to deliver nutrients from depth to the surface for phytoplankton growth. Competitive advantage will presumably accrue to small cells because they are more effective in acquiring nutrients and less susceptible to gravitational settling than large cells. Since 2004, we have discerned an increase in the smallest algae and bacteria along with a concomitant decrease in somewhat larger algae. If this trend toward a community of smaller cells is sustained, it may lead to reduced biological production at higher trophic levels.

Present and future global distributions of the marine Cyanobacteria Prochlorococcus and Synechococcus

The Cyanobacteria Prochlorococcus and Synechococcus account for a substantial fraction of marine primary production. Here, we present quantitative niche models for these lineages that assess present and future global abundances and distributions. These niche models are the result of neural network, nonparametric, and parametric analyses, and they rely on >35,000 discrete observations from all major ocean regions. The models assess cell abundance based on temperature and photosynthetically active radiation, but the individual responses to these environmental variables differ for each lineage. The models estimate global biogeographic patterns and seasonal variability of cell abundance, with maxima in the warm oligotrophic gyres of the Indian and the western Pacific Oceans and minima at higher latitudes. The annual mean global abundances of Prochlorococcus and Synechococcus are 2.9 ± 0.1 × 1027 and 7.0 ± 0.3 × 1026 cells, respectively. Using projections of sea surface temperature as a result of increased concentration of greenhouse gases at the end of the 21st century, our niche models projected increases in cell numbers of 29% and 14% for Prochlorococcus and Synechococcus, respectively. The changes are geographically uneven but include an increase in area. Thus, our global niche models suggest that oceanic microbial communities will experience complex changes as a result of projected future climate conditions. Because of the high abundances and contributions to primary production of Prochlorococcus and Synechococcus, these changes may have large impacts on ocean ecosystems and biogeochemical cycles.

Biomass of bacteria, cyanobacteria, prochlorophytes and photosynthetic eukaryotes in the Sargasso Sea

Abstract Bacteria, cyanobacteria, prochlorophytes and photosynthetic eukaryotes were enumerated in depth profiles at a station in the northern Sargasso Sea occupied for 9 days during September 1988. Carbon biomass of each group was inferred from cell abundance using conversion factors taken from the literature. Over the upper 200 m in the water column, carbon biomass occurred in the approximate proportion of 1:2:4:8 for cyanobacteria:prochlorophytes:photosynthetic eukaryotes:bacteria. Taken together, the three phytoplankton groups represented about the same amount of carbon biomass as the bacteria. This conclusion was validated by the independent measure of bulk chlorophyll a (Chl a) if the carbon:Chl a ratio was assumed to be about 44 in the nitrate-depleted layer and about 15 in the nitrate-rich layer. In reporting the biomass co-dominance of bacteria and phytoplankton, we do not deny that bacteria may dominate phytoplankton at other times and places in the oligotrophic ocean. Biomass co-dominance between these two trophic groups admits the possibility that oligotrophic bacterial assemblages may have high growth rates.

Photoacclimation of Prochlorococcus sp. (Prochlorophyta) Strains Isolated from the North Atlantic and the Mediterranean Sea

Two Atlantic (SARG and NATL1) strains and one Mediterranean (MED) strain of Prochlorococcus sp., a recently discovered marine, free-living prochlorophyte, were grown over a range of “white” irradiances (lg) and under low blue light to examine their photoacclimation capacity. All three strains contained divinyl (DV) chlorophylls (Chl) a and b, both distinguishable from “normal” Chls by their red-shifted blue absorption maximum, a Chl c-like pigment at low concentration, zeaxanthin, and [alpha]-carotene. The presence of two phaeophytin b peaks in acidified extracts from both Atlantic strains grown at high lg suggests that these strains also had a normal Chl b-like pigment. In these strains, the total Chl b to DV-Chl a molar ratio decreased from about 1 at 7.5 [mu]mol quanta m-2 s-1 to 0.4 to 0.5 at 133 [mu]mol quanta m-2 s-1. In contrast, the MED strain always had a low DV-Chl b to DV-Chl a molar ratio, ranging between 0.13 at low lg and 0.08 at high lg. The discrepancies between the Atlantic and MED strains could result from differences either in the number of light-harvesting complexes (LHC) II per photosystem II or in the Chl b-binding capacity of the apoproteins constituting LHC II. Photosynthesis was saturated at approximately 5 fg C(fg Chl)-1 h-1 or 6 fg C cell-1 h-1, and growth was saturated at approximately 0.45 d-1 for both MED and SARG strains at 18[deg]C, but saturating irradiances differed between strains. Atlantic strains exhibited increased light-saturated rates and quantum yield for carbon fixation under blue light.

Arctic Ocean microbial community structure before and after the 2007 record sea ice minimum.

Increasing global temperatures are having a profound impact in the Arctic, including the dramatic loss of multiyear sea ice in 2007 that has continued to the present. The majority of life in the Arctic is microbial and the consequences of climate-mediated changes on microbial marine food webs, which are responsible for biogeochemical cycling and support higher trophic levels, are unknown. We examined microbial communities over time by using high-throughput sequencing of microbial DNA collected between 2003 and 2010 from the subsurface chlorophyll maximum (SCM) layer of the Beaufort Sea (Canadian Arctic). We found that overall this layer has freshened and concentrations of nitrate, the limiting nutrient for photosynthetic production in Arctic seas, have decreased. We compared microbial communities from before and after the record September 2007 sea ice minimum and detected significant differences in communities from all three domains of life. In particular, there were significant changes in species composition of Eukarya, with ciliates becoming more common and heterotrophic marine stramenopiles (MASTs) accounting for a smaller proportion of sequences retrieved after 2007. Within the Archaea, Marine Group I Thaumarchaeota, which earlier represented up to 60% of the Archaea sequences in this layer, have declined to <10%. Bacterial communities overall were less diverse after 2007, with a significant decrease of the Bacteroidetes. These significant shifts suggest that the microbial food webs are sensitive to physical oceanographic changes such as those occurring in the Canadian Arctic over the past decade.

Vertical distribution of North Atlantic ultraphytoplankton: analysis by flow cytometry and epifluorescence microscopy

Abstract The vertical distribution of numerically abundant ultraphytoplankton at four stations in the central North Atlantic was established using a bench-top mercury-arc lamp flow cytometer and epifluorescence microscopy. Cyanobacteria were identified on the basis of phycoerythrin fluorescence. Red-fluorescing cells (presumably eukaryotic algae) were categorized on the basis of Coulter volume and/or size and number of chloroplasts. The data revealed a diverse community of red fluorescent cell types, including a population of very abundant (but unidentified) cells that frequently were more numerous and slightly smaller than the cyanobacteria. While eukaryotic picoplankton have been reported previously, the overwhelming numerical dominance of these cells in some cases (>90% of total) has not been emphasized. Essentially all chroococcoid cyanobacteria contained a phycoerythrin composed of phycourobilin and phycoerythrobilin. Average size of the cyanobacteria increased with depth whereas average size of the eukaryotes decreased with depth. Fluorescence per cell increased with depth for all cell types. A five-parameter mathematical equation was introduced to describe the vertical distribution of cell numbers and chlorophyll a . This permitted objective estimation of the depths at which cells or chlorophyll a reached their maximum ( z max ) and their zero ( z 0 ) values. In almost all cases, z 0 was greater than the depth of 0.1% surface irradiation and the z max for eukaryotic cells was greater than the z max for cyanobacteria.

Monitoring phytoplankton, bacterioplankton, and virioplankton in a coastal inlet (Bedford Basin) by flow cytometry

BACKGROUND To establish the prevailing state of the ecosystem for the assessment of long-term change, the abundance of microbial plankton in Bedford Basin (Nova Scotia, Canada) is monitored weekly by flow cytometry. METHODS Phytoplankton are detected by their chlorophyll autofluorescence. Those that contain phycoerythrin are designated as Synechococcus cyanobacteria or cryptophyte algae according to the intensity of light scatter. Bacteria and viruses are stained with DNA-binding fluorochromes and detected by green fluorescence. Distinction is made between bacterial and viral subpopulations exhibiting high and low fluorescence. RESULTS Time series data are presented for weekly observations from 1991 to 2000. Weekly averages are computed for the complete annual cycle of temperature, salinity, river discharge, nitrate, phosphate, silicate, chlorophyll, total phytoplankton including Synechococcus and cryptophytes, total bacteria including high and low-fluorescence subpopulations, and total viruses including high and low-fluorescence subpopulations. CONCLUSIONS The microbial biomass in the surface water of Bedford Basin is dominated by phytoplankton. The spring bloom of phytoplankton represents a maximum in algal biovolume, but not in cell number. Phytoplankton, bacteria, and viruses all attain their annual numerical maxima between the summer solstice and the autumn equinox. A vigorous microbial loop and viral shunt is envisioned to occur in the summer.

Chlorophyll distribution throughout the southeastern Mediterranean in relation to the physical structure of the water mass

The spatial distribution of chlorophyll was recorded throughout the southern part of the Levantine Basin of the eastern Mediterranean and was related to patterns of the physical structure and nutrient concentrations. Chlorophyll a concentration in the upper 200 m of the water column ranged from 9.2 to 423 ng l−1, with an overall mean of 126 ± 85.6 (SD) ng l−1. The pattern of vertical distribution of chlorophyll was close to uniform throughout the basin, with a prominent deep chlorophyll maximum (DCM) of ca. 250 ng l−1 at 90–110 m. The values we report fall at the lower end of ranges reported from other oligotrophic seas, in accordance with the ultra-oligotrophic nature of the eastern Mediterranean. Throughout the basin more than 90% of the chlorophyll at the surface was confined to particles < 10 μ in diameter and more than 60% was found in particles < 2 μ. The proportion of chlorophyll in < 2 μ particles increased with depth between the surface and the DCM, as was also confirmed by flow cytometric analysis. Discontinuities of physical and chemical features were mostly confined to depths greater than 200 m, and had little impact on the distribution of chlorophyll. An exception was an anti cyclonic eddy south of Crete, within which chlorophyll (mostly > 100 μg l−1) was more evenly distributed with depth over the upper 200 m.

Biomass and production of bacteria and phytoplankton during the spring bloom in the western North Atlantic Ocean

During the 1989 spring bloom in the western North Atlantic, we estimated the biomass and productivity of bacteria and phytoplankton at two sites (40 and 45°N) representing different water masses. At 40°N, almost all of the phytoplankton carbon could be accounted for by photosynthetic nanoplankton and picoplankton; in contrast, at 45°N, only about half was thus accounted, implying a substantial contribution by photosynthetic microplankton. At both sites, bacterial abundance was quite high (up to 2 × 109 cells l−1), and the rates of bacterial production assessed by incorporation of [3H]thymidine (up to 8 pmol l−1 h−1) and [3H]leucine (up to 240 pmol l−1h−1) were significant. Specific growth rates of bacteria based on [3H]thymidine incorporation were 0.08–0.25 day−1. Taken together, our measurements and assumptions implied a demand for primary production in the order of 16–36% over the euphotic zone or 24–78% over the upper 100 m in the water column. We conclude that ultraphytoplankton and bacteria played significant roles in the flux of carbon during the 1989 North Atlantic spring bloom.